Preparation of caprolactam

ABSTRACT

A process for the preparation of caprolactam is provided, wherein 
     a) a mixture (I) containing 6-aminocapronitrile and water is reacted in the liquid phase, in the presence of a catalyst, to give a mixture (II) containing caprolactam, ammonia, water, high-boiling components and low-boiling components, 
     b) ammonia is then removed from the mixture (II) to give a mixture (III) containing caprolactam, water, high-boiling components and low-boiling components, 
     c) water is then removed from the mixture (III) to give a mixture (IV) containing caprolactam, high-boiling components and low-boiling components, and 
     d) a solid (V) containing caprolactam is then obtained from the mixture (IV) by crystallization, the proportion by weight of caprolactam in the solid (V) being greater than in the mixture (IV).

The present invention relates to a process for the preparation ofcaprolactam, wherein

a) a mixture (I) containing 6-aminocapronitrile (“ACN”) and water isreacted in the liquid phase, in the presence of a catalyst, to give amixture (II) containing caprolactam, ammonia, water, high-boilingcomponents and low-boiling components,

b) ammonia is then removed from the mixture (II) to give a mixture (III)containing caprolactam, water, high-boiling components and low-boilingcomponents,

c) water is then removed from the mixture (III) to give a mixture (IV)containing caprolactam, high-boiling components and low-boilingcomponents, and

d) a solid (V) containing caprolactam is then obtained from the mixture(IV) by crystallization, the proportion by weight of caprolactam in thesolid (V) being greater than in the mixture (IV).

Processes for the preparation of caprolactam are generally known.

It is also generally known, for example from Ullmann's Encyclopedia ofIndustrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH,Weinheim (Germany), 1986, pages 46-48, or Kirk-Othmer, Encyclopedia ofChemical Technology, 4th Ed., Vol. 4, John Wiley & Sons, New York, 1992,page 836, that caprolactam used for the preparation of polymers musthave a purity of 99.9 to 99.94%, the main impurity conventionally beingwater in an amount of 0.04 to 0.1%. Other impurities must only bepresent in an amount of at most a few ppm.

Thus caprolactam can be prepared by a Beckmann rearrangement ofcyclohexanone oxime with sulfuric acid or oleum. After neutralization ofthe resulting mixture with ammonia, the caprolactam can be obtained fromthe ammonium sulfate formed as a by-product by extraction with anorganic solvent.

Depending on the processes for the preparation of the educts used toprepare the cyclohexanone oxime, such as cyclohexanone andhydroxylammonium sulfate, and on the oximation and rearrangementconditions, the crude caprolactam obtained by a Beckmann rearrangementcontains different types and amounts of impurities. Typical impuritiesin crude caprolactam prepared by a Beckmann rearrangement areC-methylcaprolactams, 6-methylvalerolactam and n-pentylacetamide.

Various processes are described for the purification of crudecaprolactam obtained by a Beckmann rearrangement.

According to DE-A-1253716, the crude caprolactam can be purified byhydrogenation in suspension, in the presence of a catalyst and with theaddition of an acid.

According to DE-A-1253716, the crude caprolactam can be purified byhydrogenation in suspension, in the presence of a catalyst and with theaddition of a base.

DD-A-75083 describes a process for the purification of crude caprolactamin which the crude caprolactam is first distilled and then dissolved inan organic solvent, hydrogenated in the presence of a catalyst and thentreated with an ion exchanger.

According to EP-A-411455, the important characteristic quality featuresof caprolactam can be preserved by hydrogenating the crude caprolactamcontinuously in a liquid phase process.

Crude caprolactam, obtained by the hydroformylation of 3-pentenoic acidand/or its esters to give 5-formylvaleric acid (esters) as main productsand 4- and 3-formylvaleric acid (esters) as by-products, separation ofthis (these) branched formylvaleric acid (esters) by extraction (WO97/02228) or distillation (WO 97/06126), aminating hydrogenation of5-formylvaleric acid (esters) to 6-aminocaproic acid (esters) and/or6-aminocaproic acid amide, and cyclization of 6-aminocaproic acid(esters) or 6-aminocaproic acid amide, contains other typicalimpurities.

Thus it is known e.g. from WO 99/48867, Example 1, to crystallize crudecaprolactam obtained from 5-formylvaleric acid esters, according to WO98/37063, Example 9, from mixtures of 6-aminocaproic acid,6-aminocaproic acid amide and corresponding oligomers, by the additionof 10% by weight of water. This crude caprolactam, from whichhigh-boiling and low-boiling components were not separated beforecrystallization, contained 6345 ppm of N-methylcaprolactam, 100 ppm of5-methylvalerolactam, 78 ppm of valeramide and other impurities. Thecrude caprolactam/water melt was homogenized at 50° C. and then cooledto 30° C. The crystals which precipitated out were filtered off andwashed 2 to 3 times with aqueous caprolactam. The 5-methylvalerolactamand valeramide contents were reduced to 1 ppm and theN-methylcaprolactam content to 51 ppm. 33.7 g of pure lactam wereobtained from 73.6 g of crude lactam (caprolactam yield: 45.8%). Thecharacteristic of the volatile bases (VB) was only achieved by a secondcrystallization. If high-boiling and low-boiling components wereseparated from the crude caprolactam before crystallization, accordingto WO 99/48867, Example 3, the caprolactam yield after crystallizationwas 52%.

It is further known from WO 99/65873 selectively to adsorb caprolactamfrom mixtures with 4-ethyl-2-pyrrolidone, 5-methyl-2-piperidone,3-ethyl-2-pyrrolidone and 3-methyl-2-piperidone or octahydrophenazine onadsorbents like activated carbon, molecular sieves or zeolites to givepure caprolactam after desorption. This separation of caprolactam can befollowed by crystallization from the melt or crystallization from asolvent.

It is further known to purify, by crystallization, crude caprolactamwhich, starting from 6-aminocapronitrile, is first hydrolyzed with waterto 6-aminocaproic acid, according to WO 98/37063, claim 8. Water andammonia formed by hydrolysis are then separated off, the 6-aminocaproicacid formed is cyclized and the crude caprolactam obtained iscrystallized according to WO 99/48867.

Caprolactam can also be obtained by reacting ACN with water in theliquid phase, in the presence or absence of a catalyst, with the releaseof ammonia.

In addition to caprolactam, water, ammonia and optionally another liquiddiluent, the mixture obtained in this reaction contains impuritiesboiling above caprolactam (“high-boiling components”) and impuritiesboiling below caprolactam (“low-boiling components”).

It is known from the Example in U.S. Pat. No. 496,941 that, after theseparation of water, solvent, ammonia, low-boiling component andhigh-boiling component from a mixture obtained by reacting ACN withwater and solvent, a crude caprolactam is obtained with a purity of99.5%.

Other methods of purification are described for a crude caprolactamobtained from ACN in the liquid phase since the impurities in this typeof crude caprolactam are markedly different from those in a crudecaprolactam obtained by other processes, as described in U.S. Pat. No.5,496,941.

In a first step, according to U.S. Pat. No. 5,496,941, ACN is convertedto caprolactam in the liquid phase, low-boiling components, water,ammonia and optionally other solvents are simultaneously separated off,high-boiling components are separated off to give a crude caprolactamwith a purity of 99.5%, this crude caprolactam is hydrogenated in thepresence of a catalyst, the product obtained is treated with an acidicion exchanger or sulfuric acid and the resulting product is distilled inthe presence of a base.

WO 96/20923 discloses a method of purifying crude caprolactamoriginating from the liquid phase cyclization of 6-aminocapronitrilewith water in the presence of a solvent and heterogeneous catalysts. Inthis case, crude caprolactam is first hydrogenated, then treated withacidic agents and finally distilled in the presence of alkali. Thedisadvantage of this method of purification is that three separatereaction steps are required to prepare pure caprolactam.

Said methods of purifying crude caprolactam prepared from ACN have thedisadvantage of being technically expensive and energy-intensive,especially on account of the numerous separation steps.

It is an object of the present invention to provide a process whichmakes it possible to prepare, in high purity and in a technically simpleand energy-saving manner, caprolactam which has been obtained from ACNin the liquid phase.

We have found that this object is achieved by the process defined at theoutset.

In step a), a mixture (I) containing 6-aminocapronitrile, water andoptionally liquid diluent is converted in the liquid phase, in thepresence of a solid which promotes the reaction catalytically, to amixture (II) containing caprolactam, ammonia, water, optionally liquiddiluent, high-boiling components and low-boiling components.

The ACN required for step a) can be obtained from adipodinitrile, as isgenerally known from Ullmann's Encyclopedia of Industrial Chemistry, 5thEd., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986,page 46, FIG. 8.

Particularly appropriate here is the partial catalytic hydrogenation ofadipodinitrile in the presence of ammonia as solvent and e.g. in thepresence of rhodium on magnesium oxide (U.S. Pat. No. 4,601,859), Raneynickel (U.S. Pat. No. 2,762,835, WO 92/21650) or nickel on aluminumoxide (U.S. Pat. No. 2,208,598) as a suspension catalyst or Cu—Co—Znspinel (DE-B-954416, U.S. Pat. No. 2,257,814) or iron (DE-A-42 35 466)as a fixed bed catalyst, or a process according to U.S. Pat. Nos.2,245,129, 2,301,964, EP-A-150295 or FR-A-2 029 540, or a processdescribed in U.S. Pat. No. 5,496,941.

The adipodinitrile required for this reaction is prepared industrially,e.g. by the double hydrocyanation of butadiene in the presence ofnickel-containing catalysts, and is commercially available, e.g. fromAldrich-Chemie Gesellschaft mbH & Co. KG, Steinheim, Germany.

The conversion of the mixture (I) to the mixture (II) can be carried outaccording to FR-A-2029540 in the presence of catalysts, the catalystsused being metallic Zn or Cu powder or the oxides, hydroxides, halidesor cyanides of rubidium, lead, mercury or the elements of atomic numbers21 to 30 or 39 to 48. The catalysts described are used as suspensioncatalysts in stirred autoclaves operated batchwise.

A process described in U.S. Pat. Nos. 5,646,277, 5,739,324 or WO59/14665 is particularly preferred as step a).

In these processes, the reaction is carried out in the liquid phase attemperatures generally of 140 to 320° C., preferably of 160 to 280° C.;the pressure ranges generally from 1 to 250 bar, preferably from 5 to150 bar, it being necessary to ensure that the reaction mixture ispredominantly liquid under the conditions used. The residence timesrange generally from 1 to 120 min, preferably from 1 to 90 min andparticularly preferably from 1 to 60 min. Residence times of 1 to 10 minhave proved totally satisfactory in some cases.

The reaction can be carried out batchwise or, preferably, continuously.Suitable reactors are a stirred tank, an autoclave or, preferably, amultitube fixed-bed reactor. Combinations of such reactors are alsopossible.

The amount of water used is generally at least 0.1 mol, preferably 0.5to 100 mol and particularly preferably 1 to 20 mol per mol of ACN.

Advantageously, the ACN is used in the form of a 1 to 50% by weight,especially 5 to 50% by weight and particularly preferably 5 to 30% byweight solution in water (in which case the solvent is simultaneously areactant) or in mixtures containing water and a liquid diluent. Examplesof liquid diluents which may be mentioned are alkanols such as methanol,ethanol, n- and i-propanol and n-, i- and t-butanol, polyols such asdiethylene glycol and tetraethylene glycol, hydrocarbons such aspetroleum ether, benzene, toluene and xylene, lactams such aspyrrolidone or caprolactam, alkyl-substituted lactams such asN-methylpyrrolidone, N-methylcaprolactam or N-ethylcaprolactam, andcarboxylic esters, preferably those of carboxylic acids having from 1 to8 C atoms. Ammonia can also be present in the reaction. Of course, it isalso possible to use mixtures of organic liquid diluents. Mixtures ofwater and alkanols in a water/alkanol weight ratio of 1-75/25-99,preferably 1-50/50-99, have been shown to be particularly advantageousin some cases.

In principle, it is equally possible to use the ACN as reactant andsolvent at the same time.

Examples of heterogeneous catalysts which can be used are acidic, basicor amphoteric oxides of the elements of main group II, III or IV of thePeriodic Table, such as calcium oxide, magnesium oxide, boron oxide,aluminum oxide, tin oxide or silicon dioxide in the form of pyrogenicsilicon dioxide, silica gel, kieselguhr, quartz or mixtures thereof, andalso oxides of metals of subgroups II to VI of the Periodic Table, suchas amorphous titanium oxide in the form of anatase or rutile, zirconiumoxide, zinc oxide, manganese oxide or mixtures thereof. It is alsopossible to use lanthanide and actinide oxides such as cerium oxide,thorium oxide, praseodymium oxide, samarium oxide, a rare earth mixedoxide or mixtures thereof with the abovementioned oxides. Examples ofother possible catalysts are:

vanadinium [sic] oxide, niobium oxide, iron oxide, chromium oxide,molybdenum oxide, tungsten oxide or mixtures thereof. Mixtures of saidoxides with one another are also possible. Some sulfides, selenides andtellurides, such as zinc telluride, tin selenide, molybdenum sulfide,tungsten sulfide and the sulfides of nickel, zinc and chromium, can alsobe used.

The abovementioned compounds can be doped with, or contain, compounds ofmain groups I and VII of the Periodic Table.

Other suitable catalysts which may be mentioned are zeolites, phosphatesand heteropolyacids, as well as acidic and alkaline ion exchangers likeNafion.

These catalysts can optionally contain up to 50% by weight in each caseof copper, tin, zinc, manganese, iron, cobalt, nickel, ruthenium,palladium, platinum, silver or rhodium.

Particularly preferred catalysts which have very high conversions,yields, selectivities and working lives under the above-describedreaction conditions are heterogeneous catalysts based on titanium oxide,zirconium oxide, cerium oxide and aluminum oxide, especially titaniumdioxide. They can be used in the form of powders, chips, grit, strandsor tablets (produced by compression). The form of the oxides normallydepends on the requirements of the particular reaction procedure, powderor chips being used in suspension. In the fixed bed procedure, it isconventional to use tablets or strands with diameters of between 1 mmand 10 mm.

Aluminum oxide is suitable in any modifications which can be obtained byheating the aluminum hydroxide precursor compounds (gibbsite, boehmite,pseudoboehmite, bayerite and diaspore) at varying temperatures. Theseinclude especially gamma- and alpha-aluminum oxide and mixtures thereof.

The oxides can be used in the pure form (content of the individualoxide >80% by weight), as a mixture of the abovementioned oxides, inwhich case the sum of the abovementioned oxides should be >80% byweight, or as a supported catalyst, in which case the above-mentionedoxides can be applied to a mechanically and chemically stable support,usually with a high surface area.

The pure oxides may have been prepared by precipitation from aqueoussolutions, e.g. titanium dioxide by the sulfate process, or by otherprocesses such as the pyrogenic preparation of fine aluminum oxide,titanium dioxide or zirconium dioxide powders, which are commerciallyavailable.

A choice of several methods is available for the preparation of mixturesof the different oxides. The oxides, or their precursor compounds whichcan be converted to the oxides by calcination, can be prepared e.g. byjoint precipitation from solution. A very good distribution of the twooxides used is generally obtained by this method. The oxide or precursormixtures can also be obtained by precipitation of one oxide or precursorin the presence of the second oxide or precursor present as a suspensionof fine particles. Another method consists in mechanically mixing theoxide or precursor powders, it being possible for this mixture to beused as a starting material for the production of strands or tablets.

In principle, supported catalysts can be prepared by any of the methodsdescribed in the literature. Thus the oxides can be applied to thesupport in the form of their sols simply by impregnation. The volatileconstituents of the sol are conventionally removed from the catalyst bydrying and calcination. Such sols are commercially available fortitanium dioxide, aluminum oxide and zirconium dioxide.

Another possible way of applying layers of the active oxides consists inhydrolyzing or pyrolyzing organic or inorganic compounds. Thus a ceramicsupport can be coated with a thin layer of titanium dioxide byhydrolyzing titanium isopropylate or other Ti alkoxides. Other suitablecompounds are TiCl₄, zirconyl chloride, aluminum nitrate and ceriumnitrate, inter alia. Suitable supports are powders, strands or tabletsof said oxides themselves or of other stable oxides like silicondioxide. The supports used can be in a macroporous form in order toimprove the material transport.

In step b), ammonia is removed from the mixture (II) to give a mixture(III) containing caprolactam, water, optionally liquid diluent,high-boiling components and low-boiling components.

In principle, the separation of the ammonia from the mixture (II) can beeffected by methods known per se for the separation of materials, suchas extraction or, preferably, distillation, or a combination of suchmethods.

The distillation can advantageously be carried out at bottomtemperatures of 60 to 220° C., especially of 100 to 220° C. Thepressure, measured at the top of the distillation performance [sic], isconventionally set at 2 to 30 bar absolute.

Suitable apparatuses are those conventionally used for distillation, forexample the ones described in Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages870-881, such as sieve-plate columns, bubble-cap columns or packedcolumns.

The distillation can be carried out in several columns, such as 2 or 3,but advantageously in a single column.

In step c), water and optionally liquid diluents are removed from themixture (III) to give a mixture (IV) containing caprolactam,high-boiling components and low-boiling components.

If a liquid diluent has been used in step a), water and liquid diluentcan be separated off simultaneously in step c) or the water can beseparated off before or after the liquid diluent.

In principle, the water can be separated from the mixture (III) bymethods known per se for the separation of materials, such asextraction, crystallization or, preferably, distillation, or acombination of such methods.

The distillation can advantageously be carried out at bottomtemperatures of 50 to 250° C., especially of 100 to 230° C.

Suitable apparatuses are those conventionally used for distillation, forexample the ones described in Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages870-881, such as sieve-plate columns, bubble-cap columns or packedcolumns.

The distillation can be carried out in several columns, such as 2 or 3,but advantageously in a single column.

A heat-coupled multistage separation of the water and optionally theliquid diluent is particularly preferred.

Before the mixture (IV) is introduced into step d), it is appropriate toseparate off the low-boiling component [sic] and high-boiling component[sic], advantageously only the high-boiling components, especiallyneither the low-boiling component [sic] nor the high-boiling component[sic] and particularly advantageously only the low-boiling componentsfrom the mixture (IV).

If the low-boiling components and high-boiling components are separatedfrom the mixture, the low-boiling components can be separated offbefore, after or together with the high-boiling components.

In the case where the low-boiling component [sic] and high-boilingcomponent [sic], or only the high-boiling component [sic], or only thelow-boiling component [sic], are separated off, the separation can beeffected in principle by methods known per se for the separation ofmaterials, such as extraction, crystallization or, preferably,distillation, or a combination of such methods.

The distillation can advantageously be carried out at bottomtemperatures of 50 to 250° C., especially of 100 to 230° C. Thepressure, measured at the top of the distillation performance [sic], isconventionally set at 1 to 500 mbar absolute, preferably 5 to 100 mbarabsolute.

Suitable apparatuses are those conventionally used for distillation, forexample the ones described in Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages870-881, such as sieve-plate columns, bubble-cap columns or packedcolumns.

The distillation for separating off the low-boiling components can becarried out in several columns, such as 2 or 3, but advantageously in asingle column.

The distillation for separating off the high-boiling components can becarried out in several columns, such as 2 or 3, but advantageously in asingle column.

In step d), a solid (V) containing caprolactam is obtained from themixture (IV) by partial crystallization, the proportion by weight ofcaprolactam in the solid (V) being greater than in the mixture (IV).

The sum of the contents of high-boiling and low-boiling components, notincluding water and organic diluents, in the mixture (IV) used in stepd) is advantageously at least 100 ppm by weight, preferably 200 ppm byweight, particularly preferably at least 500 ppm by weight andespecially at least 1000 ppm by weight, based on the mixture (IV).

The crystallization can be effected batchwise or continuously.

The crystallization can be effected with the addition of an aid such asan organic or inorganic liquid diluent, for example water, butpreferably without the addition of an aid.

The crystallization can be effected in one or more stages, such as two,three or four stages, preferably one stage. In another preferredembodiment of the invention, the crystallization can be effected asfractional crystallization.

In the case of fractional crystallization, all the stages producing acrystalline product (caprolactam) which is purer than the initial crudeproduct (crude caprolactam) are conventionally called purificationstages, and all the other stages are conventionally called refiningstages. It is advisable here to operate multistage processes accordingto the counter-current principle, whereby, after the crystallization ineach stage, the crystalline product is separated from the remainingliquid phase (“mother liquor”) and transferred to the appropriate stagewith the next highest degree of purity, the crystallization residuebeing transferred to the appropriate stage with the next lowest degreeof purity.

Advantageously, the temperature of the solution or melt duringcrystallization is not higher than the melting point of caprolactam (70°C.) and is preferably between −10 [sic] and the melting point ofcaprolactam and especially between 20 [sic] and the melting point ofcaprolactam. The solids content in the crystallizer is conventionallybetween 0 and 70 g, preferably between 30 and 60 g, per 100 g of charge.

In another advantageous embodiment of the invention, the crystallizationis effected in apparatuses in which the crystals grow on cooled surfacesin the crystallization apparatus, i.e. are fixed in the apparatus (e.g.layer crystallization process from Sulzer Chemtech (Switzerland) orstatic crystallization process from BEFS PROKEM (France)).

The crystallization can also be effected by cooling apparatus walls orby evaporating a solution of the crude caprolactam under reducedpressure. Five to 30% by weight solutions of crude caprolactam in aliquid diluent, especially water, are particularly suitable for thispurpose.

In the case of crystallization by cooling, the heat can be removed viascraped wall chillers connected to a stirred tank or an unstirredvessel. The crystal suspension can be circulated by means of a pump. Afurther possibility is to remove the heat via the wall of a tank with awall-fitted stirrer. Another preferred embodiment of crystallization bycooling is the use of cooling disk crystallizers, e.g. thosemanufactured by Gouda (Holland). In another suitable variant ofcrystallization by cooling, the heat can be removed via conventionalheat exchangers (preferably shell-and-tube or parallel-plate heatexchangers). In contrast to scraped wall chillers, tanks withwall-fitted stirrers or cooling disk crystallizers, these apparatuses donot possess a device for preventing layers of crystals from forming onthe heat-transfer surfaces. If, during operation, a situation is reachedwhere the resistance to heat transition due to the formation of layersof crystals becomes excessive, the conventional procedure is to switchover to a second apparatus. During the operating period of the secondapparatus, the first apparatus can be regenerated (preferably by meltingthe layer of crystals or flushing the apparatus with unsaturatedsolution). If the resistance to heat transition in the second apparatusbecomes excessive, the procedure is to switch back to the firstapparatus, and so on. This variant can also be operated with more thantwo apparatuses in alternation. The crystallization can also be effectedby conventional evaporation of the solution under reduced pressure.

The solid-liquid separation methods known per se are suitable forseparating the mother liquor from the caprolactam which has crystallizedout.

In one preferred embodiment of the invention, the crystals can beseparated from the mother liquor by filtration and/or centrifugation.Advantageously, the filtration or centrifugation can be preceded bypreliminary concentration of the suspension, for example by means of oneor more hydrocyclones. Centrifuges known per se, which operate batchwiseor continuously, are suitable for the centrifugation. It is mostadvantageous to use pusher centrifuges, which can be operated in one ormore stages. Screen-conveyor centrifuges or helical-conveyor centrifuges(decanters) are also suitable. The filtration can advantageously beeffected by means of suction filters, which can be operated batchwise orcontinuously and with or without a stirrer, or by means of belt filters.The filtration can generally be carried out under superatmosphericpressure or under reduced pressure.

During and/or after the solid-liquid separation, provision can be madefor further process steps to increase the purity of the crystals orcrystal cake. In one particularly advantageous embodiment of theinvention, the separation of the crystals from the mother liquor isfollowed by washing and/or sweating of the crystals or crystal cake inone or more stages.

In the case of washing, the amount of washing liquor should preferablybe between 0 and 500 g per 100 g of crystalline product, preferablybetween 30 and 200 g per 100 g of crystalline product.

Suitable washing liquors are organic or inorganic liquids or mixtures ofsuch liquids, examples of preferred washing liquors being

a) in the case where a liquid diluent has been used in thecrystallization in step d), said liquid diluent,

b) a melt of a crystalline product obtained in a crystallization stageof step d),

c) a mother liquor obtained in a crystallization stage of step d), or

d) a melt of an educt used in a crystallization stage of step d).

The washing can be effected in apparatuses conventionally used for thispurpose. It is advantageous to use wash columns, in which the separationof the mother liquor and the washing take place in one apparatus,centrifuges, which can be operated in one or more stages, or suctionfilters or belt filters. The washing can be effected on centrifuges orbelt filters in one or more stages, it being possible for the washingliquor to be conveyed in countercurrent to the crystal cake.

Particularly in the case of crystallization without the addition of anaid, the washing liquor can be recycled into the crystallization,optionally after impurities have been separated off.

Sweating is conventionally understood as meaning a local melting ofcontaminated regions. The amount of sweating should advantageously be0.1 to 90 g of melted crystalline product per 100 g of crystallineproduct prior to sweating, preferably 5 to 35 g of melted crystallineproduct per 100 g of crystalline product. It is particularly preferredto carry out the sweating on centrifuges or belt filters. It may also beappropriate to combine washing and sweating in one apparatus.

Particularly in the case of crystallization without the addition of anaid, the mother liquor can be recycled into the crystallization,optionally after impurities have been separated off.

Caprolactam can be obtained in a purity of at least 99.90% by weight,preferably 99.90 to 99.99% by weight, by the present process.

The caprolactam obtainable by the process according to the invention canbe used for the preparation of polyamides like polycaprolactam.

EXAMPLES Example 1a

Preparation of Crude Caprolactam

The purification sequence was carried out with crude caprolactamobtained according to WO 95/14664 by the cyclization of a 10% ethanolicsolution of 6-aminocapronitrile (ACN) in the presence of 2.5 mol ofwater per mol of ACN:

A solution of 6-aminocapronitrile (ACN) in water and ethanol (10% byweight of ACN, 4.0% by weight of water, remainder: ethanol) was passedat 100 bar into a heated tubular reactor of capacity 25 ml (diameter 6mm; length 800 mm) packed with titanium dioxide (anatase) in the form of1.5 mm strands, the reaction temperature being 245° C. and the residencetime being 30 min. The product stream leaving the reactor was analyzedby gas chromatography: conversion: 100%, yield: 87%.

The reaction discharge was freed of high-boiling and low-boilingcomponents by fractional distillation. According to gas chromatographicanalysis, the resulting crude caprolactam had a purity of 99.90%.

Example 1b

Purification of crude caprolactam by crystallization 492 g of liquidcrude caprolactam from Example 1a were crystallized on the flat cooledbottom plate of a stirred vessel.

The bottom plate of the vessel was cooled from an initial temperature of75° C. at a rate of −50 K/h. After the temperature had fallen below themelting point of the crude caprolactam melt, a constantly growing layerof pure caprolactam crystals formed on the cooling surface. To ensure agood material transition between the liquid and solid phases during thegrowth process, the crude caprolactam melt was stirred with a vanestirrer at 500 rpm. When the weight of crystal mass had reached 378 g(yield 77% by weight), the cooling was stopped, the stirrer was switchedoff and the stirred vessel was rotated through 180° so that the bottomwas now pointing upward, thereby separating the residual melt enrichedin impurities (mother liquor, 23% by weight) from the layer ofcaprolactam crystals adhering to the bottom plate. The mass ofcaprolactam crystals could be removed by dismantling the bottom plate ofthe vessel.

Table 1 shows the properties of the crude caprolactam of Example 1a andthe crystals and mother liquor of Example 1b.

Table 2 shows the conventional required specification of commerciallyavailable caprolactam, together with the corresponding values of thecrude lactam of Example 1a and the caprolactam purified according toExample 1b (“crystals”).

TABLE 1 Amount [% by weight] Capro ACN HMD EAC EECL ZECL Crude 100 99.90 52 ppm    31 101  35 22 ppm lactam ppm ppm ppm Crystals 77 99.991  4ppm  <1  5  12  7 ppm ppm ppm ppm Mother 23 99.832 205 ppm   122 431 10581 ppm liquor ppm ppm ppm

TABLE 2 Free Volatile bases bases Color [meq/kg] [meq/kg] Absorbancenumber PAN Specification 0.1 0.5 0.05 5 4 Crude lactam 1.3 0.73 0.18 7.27.8 Crystals 0.09 0.34 0 0 1.6

Abbreviations:

ACN: 6-aminocapronitrile HMD: hexamethylenediamine EAC: ethyl6-aminocaproate EECL: E-alpha-ethylidenecaprolactam ZECL:Z-alpha-ethylidenecaprolactam

Methods of Analysis

Free Bases

To determine the free bases, 150 ml of CO₂-free distilled water throughwhich nitrogen had been passed were adjusted to exactly pH 7.0 with 0.01N sodium hydroxide solution, and 50±0.1 g of caprolactam were added. Themixture was then titrated to pH 7.0 with 0.01 N hydrochloric acid at 25°C. The proportion of free base could then be calculated according to theformula below, A (ml) denoting the volume of 0.01 N hydrochloric acidconsumed. Free bases=0.01×A×1000/50=0.2×A meq/kg.

Volatile Bases (VB)

The volatile bases were determined according to ISO standard 8661(“Caprolactam for industrial use—Determination of volatile basescontent”).

The volatile bases were liberated from the sample by distillation in analkaline medium (Kjeldahl apparatus), trapped in 0.01 N hydrochloricacid and determined by titration with 0.01 N sodium hydroxide solution.The initial weight of caprolactam sample was 20±0.1 g.${VB} = {\frac{\left( {B - A} \right) \times 0.01}{20} \times 1000\quad {{meq}/{kg}}}$

A=volume of 0.01 N sodium hydroxide solution consumed

B=volume of 0.01 N sodium hydroxide solution consumed for a blankdetermination

Absorbance

The absorbance was carried out according to ISO standard 7059(“Caprolactam for Industrial Use—Determination of Absorbance at aWavelength of 290 nm”).

Color Number

The color number was determined according to ISO standard 8112.

50±0.1 g of caprolactam were dissolved in 50 ml of distilled water in a250 ml Erlenmeyer flask and left to stand until the air bubbles haddisappeared. In two reference cuvettes (1=5 cm), the difference inabsorbance A₂₉₀ between water and CL/water was determined at λ=390 [sic]nm.

Color number=150×A ₂₉₀ (rounded to nearest integer)

Permanganate Absorption Number (PAN)

The PAN was determined according to ISO standard 8660.

This was done by adding identical amounts of 0.01 N potassiumpermanganate solution to a 3% (m/m) aqueous solution of caprolactam andto a blank sample (distilled water). After 10 minutes the absorbances Aof both the caprolactam sample and the blank sample were compared at 420nm. The permanganate absorption number is calculated according to thefollowing formula:

PAN (PI)=(A−A ₀)₄₂₀×100/3

We claim:
 1. A process for the preparation of caprolactam, wherein a) a mixture (I) containing 6-aminocapronitrile and water is reacted in the liquid phase, in the presence of a catalyst, to give a mixture (II) containing caprolactam, ammonia, water, high-boiling components and low-boiling components, b) ammonia is then removed from the mixture (II) to give a mixture (III) containing caprolactam, water, high-boiling components and low-boiling components, c) water is then removed from the mixture (III) to give a mixture (IV) containing caprolactam, high-boiling components and low-boiling components, and d) a solid (V) containing caprolactam is then obtained from the mixture (IV) by crystallization, the proportion by weight of caprolactam in the solid (V) being greater than in the mixture (IV).
 2. A process as claimed in claim 1 wherein the mixture (I) additionally contains an organic liquid diluent.
 3. A process as claimed in claim 2 wherein the liquid diluent is removed in step c) before, during or after the separation of the water from the mixture (III).
 4. A process as defined in claim 1, wherein the low boilers are separated between steps c) and d).
 5. A process as defined in claim 1, wherein the high boilers are separated between steps c) and d).
 6. A process as defined in claim 1, wherein the low boilers and high boilers are separated between steps c) and d).
 7. A process as defined in claim 1, wherein the sum of the contents of high boilers and low boilers in the mixture (IV) used in step d), without water and any liquid diluents still present, is at least 100 ppm by weight.
 8. A process as defined in claim 1, wherein crystallization in step d) is carried out in the absence of auxiliary agents.
 9. A process as defined in claim 1, wherein crystallization in step d) is carried out on a cooled surface, on which solid (V) grows.
 10. A process as defined in claim 1, wherein the mother liquor obtained after crystallization in step d) is mixed with mixture (IV) and recycled to step d).
 11. A process as defined in claim 1, wherein crystallization in step d) is carried out batchwise. 